1. Field of Invention
This invention relates generally to current-to-frequency converters which produce pulses whose repetition rate depends on the current level, and more particularly to a converter of this type whose required operating power is extremely low so that the converter can be battery operated.
2. Status of Prior Art
In a typical industrial process control loop that is subject to disturbances, a controIler acts in conjunction with other devices to maintain the process variable at a desired value. The variable factor controlled may be flow rate, pressure, viscosity, liquid level, or any other process variable. Each process variable is sensed by a transducer such as a pressure gauge, a magnetic flowmeter, or a temperature-sensitive element which converts the sensed process variable into a corresponding electrical analog signal. In many transducers of this type, the output yielded thereby lies in the 4 to 20 milliampere range, this being the standard range in the process control field.
In operation, the electronic controller receives both the process variable and a set point signal, and compares these electrical values to produce an output signal that reflects the deviation of the process variable from the set point. This output signal is applied to the final control element in the loop to directly or indirectly govern the process variable. Thus the process variable input signal to a controller may be derived from a flowmeter whose reading is converted into a corresponding electrical value, and the output signal from the controller may be impressed on a flow-regulating valve which is adjusted to cause the flow rate to conform to the set point.
In recent years, process control systems have been developed which include a multiplicity of process control loops and a digital computer common thereto functioning as a controller for all of the loops. To obviate the need for complex wiring arrangements, the exchange of information between the loop sensors and the final control elements in the respective loops is effected by a timesharing multiplexing technique.
Various forms of multiplexer networks associated with a multiplicity of process control loops and operating in conjunction with a digital computer which regulates each loop, are disclosed in the Nabi U.S. Pat. No. 3,760,374 (Foxboro), the Smith U.S. Pat. No. 3,566,355 (Motorola), the Ries et al. U.S. Pat. No. 3,526,757 (Owens-Corning), and the Jacques et al. U.S. Pat. No. 3,479,493 (IBM). In the multiplexer coupled analog input technique, each control loop transducer is connected by switch to a multiplexer bus. The switches for the several transducers are sequentially actuated so that successively applied to the bus are the analog signals from the loop transducers. The bus is connected through an amplifier to an analog-to-digital converter whose digital signals are applied to the digital computer which governs the final control elements in the loops.
Thus in an era of microprocessors and microcomputers, a common requirement where the input data is in analog current or voltage form, is an A to D converter for converting the analog value into a digital format which can be processed by digital equipment.
The problem to which the present invention is directed is that which arises where the transducer which serves the process variables is at a remote field location lacking power line facilities, and where it is then necessary to operate the A to D converter with battery power. One must under these circumstances provide a converter which draws very little current from the low voltage battery supply; otherwise the battery will be exhausted in a relatively short period.
It is also essential that the converter so operated have an accuracy comparable to that of conventional A to D converters which draw their operating power from a power line.